HDFS-12889. Addendum patch to add missing file.

(cherry picked from commit 6cca5b3bcb440095f12d3eda88101fa250ac000a)

hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/router/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /